Optimizing Compressed Air Systems

Although it is often viewed as an essentially free resource, compressed air (CA) is anything but free. In fact, in many industrial plants, air compressors consume more energy than any other single end use. And once the air is compressed to the desired pressure, it often has to be dried and cooled before it is sent through the distribution system to the end use, requiring even more energy (Figure 1). According to the U.S. Department of Energy (DOE), CA accounts for 10 percent of industrial electricity consumption.

Figure 1: Compressed air system diagram

This shows a typical compressed air system with compression, cooling, storage, and distribution equipment.

Fortunately, most industrial facilities have many opportunities to improve CA-system efficiency through measures with rapid paybacks. A DOE study found that, in small to midsize industrial facilities, about 15 percent of CA-system energy use can be saved through actions that pay for themselves in less than two years. In larger facilities, payback periods can be even shorter, with savings that can range from 30 to 60 percent of existing system usage. Although a comprehensive optimization of your CA system usually requires the services of a trained professional, there are a number of very cost-effective steps you can take today, without the aid of an expert, to make your system more efficient. These steps include reducing system pressure to the minimum necessary to get the job done, identifying and fixing leaks, eliminating inappropriate uses of CA, improving system control, and implementing a regular maintenance plan.

The concepts presented briefly here (and many more) are discussed in greater detail in Improving Compressed Air System Performance: A Sourcebook for Industry, [1] published by the DOE.

Reduce system pressure

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Reducing system pressure to the minimum that is absolutely necessary is frequently the most cost-effective and quickest payback opportunity for energy savings in a CA system. It should be your first step in system optimization.

The goal of a CA system is to deliver air at the lowest appropriate pressure for system needs while supporting spikes in demand with stored CA. However, it is not unusual to find CA systems operating at higher pressures than necessary. This can happen for a variety of reasons, such as inadequate information about end-use requirements, changes to production demands over time, or suboptimal air-storage capacity. Operating your CA system above the minimum necessary pressure is wasteful for three reasons. First, the higher the air pressure needs to be, the more compressor energy it takes to reach appropriate levels. One rule of thumb is that for systems operating at about 100 pounds per square inch (psi), every increase of 2 psi raises input power to the compressor by 1 percent at full flow. The opposite is also true—one DOE case study found that a pressure drop of 2 psi resulted in a 1 percent reduction in power. Second, for unregulated CA end uses, the volume of air consumed depends on the air pressure—as pressure gets higher, more air is consumed, so CA requirements and energy costs can often be reduced significantly without affecting performance by simply reducing system pressure. Similarly, the higher the system pressure, the more air is driven through the leaks that are common to CA systems. For example, at 80 psi, about 21.4 cubic feet per minute (cfm) of air will flow through a one-eighth-inch diameter leak. At 100 psi, that flow would increase by over 20 percent to 26 cfm, wasting thousands of dollars annually.

Reducing pressure without affecting production processes requires that you be aware of the minimum pressure at which each CA end use can operate. If you find that none of the CA end uses in your plant require the pressure being delivered, you can save energy at almost no cost by dialing back compressor discharge pressure in small increments to the minimum that maintains satisfactory equipment performance.

Sometimes, elevated pressures are maintained to compensate for unacceptable pressure drops that would otherwise occur due to large, intermittent CA consumers on the same distribution system. In such cases, adding secondary storage capacity at or near the point of use is an inexpensive solution to smooth out systemwide pressure fluctuations. Section 5.F of the DOE’s Sourcebook describes how to calculate the volume of secondary storage needed for a particular application.

Another common reason CA systems operate at unnecessarily high pressure is that one or more end uses require it. In such cases, it can often be profitable to install either a booster compressor with local storage or a separate compressor and air distribution system dedicated to these high-pressure end uses. Doing so allows the rest of the plant to operate at lower pressure and can result in dramatic energy savings.

Finally, excessive pressure drop through the components of the air treatment and distribution system can necessitate higher compressor discharge pressure to ensure that the pressure will be adequate by the time it gets to the end use. In a well-designed and -maintained CA system, the pressure at the end use should be at least 90 percent of the initial compressor discharge pressure. Virtually every component of the CA system downstream of the compressor can be a source of pressure drop, such as dryers and filters on the supply side and undersized distribution piping, equipment hoses, disconnect couplings, filters, regulators, or lubricators on the demand side. If you find pressure at the end use to be significantly below 90 percent of compressor discharge, work upstream one component at a time to identify where the major pressure drops are occurring. When specifying or replacing this equipment, always ask manufacturers to provide information on pressure drop at the maximum anticipated flow rate and select equipment that minimizes it. Also, be sure to clean or replace filter elements regularly.

Finding the optimum pressure can also be an iterative process. Some of the other suggestions provided here, such as eliminating leaks, can result in higher air pressures at the end use. It’s important to measure and record air pressure at each end use, both before and after you make any improvements to the system, because you may be able to gain even greater energy savings by further reducing system pressure.

Find and eliminate leaks

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Air leaks can waste a considerable portion of a compressor’s output. According to the DOE, it’s not unusual for leaks to consume 20 to 30 percent of compressor output, which can add up to thousands of dollars per year in unnecessary electricity costs. Moreover, leaks reduce system pressure, which can cause air tools to operate inefficiently, which could in turn affect production. Finally, all the air that leaks out must be replaced by the compressor, causing compressors to run for longer periods, thus reducing their lifetime.

The first step in dealing with leaks is to estimate the amount of leakage. There are two straightforward ways to do this, but both methods must be done while production is shut down. For systems that have on/off or load/unload controls, allow the compressor to bring the system up to the pressure setpoint. Then allow the CA system to run through several cycles (more cycles will give you greater accuracy) as the pressure drops due to leakage and the compressor kicks on or loads up to bring pressure back to the setpoint. On each cycle, record the amount of time that the compressor is on or loaded. The ratio of the on or loaded time to the total time of the test is the leakage fraction.

For systems that have other types of capacity control, leakage can be estimated by noting the time it takes for system pressure to drop from its setpoint to one-half of setpoint pressure with the compressor off and no production activity. The leakage rate (L), measured in cfm, is determined by the equation:

L = [(PS x V) ÷ (2 x T x 14.7)] x 1.25

where PS is setpoint pressure in pounds per square inch gauge (psig); V is the total system volume, including all storage and distribution piping, in cubic feet; T is the time in minutes it takes for the system to drop to one-half the setpoint pressure; 14.7 is the conversion value from psig to atmospheric pressure; and 1.25 is a correction factor. By comparing this leakage rate to the total volume of CA delivered, you can estimate the fraction of CA costs that are wasted by leaks. Systems with leakage rates of 10 percent or more can likely be improved; your goal should be to bring the leakage rate down well below 10 percent.

The next step is to find and eliminate the leaks. Many leaks are audible and easily located, especially during nonproduction periods. For others, an ultrasonic leak detector (available from a number of manufacturers) is an effective tool. Once you’ve found a leak, eliminating it is often just a matter of tightening the connection, but sometimes it will be necessary to open a joint, clean the threads, and apply proper thread sealant. In some cases you may find that you need to remove and replace faulty equipment such as damaged hoses and drain valves. Finally, ensure that the air supply to all equipment is shut off when that equipment is not in use. Solenoid valves are available that will automate that process.

Once you’ve completed your leak hunt and eliminated as many leaks as possible, reevaluate the leakage rate to determine the impact you’ve had on the system and to estimate the resulting savings. Also reevaluate the system pressure during normal plant operation—you may find that you are now able to further reduce the compressor discharge setpoint and gain additional savings.

Identify and eliminate inappropriate uses of compressed air

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Compressed air is used in a wide variety of applications that could be performed more efficiently (and in many cases more effectively) using different methods. Table 1 lists several common misapplications of compressed air, along with better alternatives. You’ll find a more comprehensive discussion in the DOE’s Sourcebook.

Table 1: Common inappropriate uses for compressed air

This table lists some of the common misapplications of compressed air and provides more-efficient alternatives.

[2]

Although up-front capital investment will be necessary to eliminate some inappropriate applications, such as procuring the appropriate alternative equipment, performing these processes with compressed air is so inefficient that the required investment will usually be repaid quickly. Eliminating inappropriate uses will reduce CA consumption and may allow you to shut down one or more compressors entirely. This can save capital in the future as well—should expanded production require additional compressor capacity, you’ll have it ready and waiting.

Improve system control

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Optimizing CA system control normally requires careful analysis by a trained professional. However, there is one symptom of a poorly controlled CA system that even the untrained eye can identify. All air compressors operate most efficiently when running at full capacity. Unfortunately, it is very rare for CA demand to precisely match the full load output of its air compressor. Much more frequently, some type of control system operates one or more compressors to match CA supply with demand. There are numerous types of control technologies and strategies, some of which are specific to the type of compressor in use. But regardless of compressor type, system efficiency will be optimized by operating as many compressors as necessary at full load and operating only one “trim” compressor with good partial-load efficiency to match supply with demand. The most efficient compressors should be chosen for full-load operation. If you find that you’ve got two or more compressors simultaneously running under partial load that are feeding into the same CA system, it’s probably time to revamp your control strategy.

Perform regular maintenance

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Like all other building systems, CA systems require regular maintenance to keep operational efficiency maximized while minimizing unscheduled shutdowns due to equipment failure. Implementing a preventative maintenance plan with actions like replacing filters and fluids, adjusting belts, inspecting components, and identifying and repairing leaks will help facilities avoid increased energy consumption. Malfunctioning CA equipment can lead to drops in system efficiency as well as full system shutdowns, which can result in production downtime. Manufacturers of CA systems provide maintenance procedures and schedules for each component.

Facilities should also consider benchmarking the CA system. Recording baseline values for system parameters such as power consumption, pressure, airflow, and temperature will provide data that maintenance staff can use to identify problems early on—for example, a steady increase in power use without increasing pressure or air flow indicates a drop in system efficiency.

Resources for further information

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As noted above, comprehensive optimization of CA systems usually requires the services of a CA expert. Because most CA systems have opportunities for cost-effective savings, hiring a trained professional to perform an audit of your CA system is a good idea. But whether you’re looking to hire an expert or planning to work on your CA system yourself, you’ll find the following resources to be helpful:

Compressed Air Challenge (CAC). The CAC is a collaboration of compressed air manufacturers, distributors, and their associations; industrial users; consultants; state research and development agencies; energy-efficiency organizations; and utilities. On its website, you can learn about upcoming training opportunities and download documents covering a wide variety of CA concepts as well as case studies that demonstrate them. Of particular note, the CAC-produced manual, Best Practices for Compressed Air Systems: Second Edition, can be ordered here. This comprehensive and easy-to-follow book compiles information on most aspects of CA-system maintenance.